Electric device with electron barrier



Patented Feb. 4, 1947 ELECTRIC DEVICE WITH ELECTRON BARRIER Edwin Joseph Merrell, Eastchester, N. Y., assignor to Phelps Dodge Copper Products Corporation, Dover, Dei., a corporation of Delaware Application February 5, 1943, Serial No. 474,886

2 Claims. 1

This invention relates to a means for enabling the full electric strength of an insulating medium of an electrical device to be realized, without loss due to local surface weaknesses. Its application is to electric cables, condensers, and insulating windings of other electric devices; more particularly it consists of a sheet of novel material applied to the surfaces of the insulating medium.

The characteristics of this electron barrier are: (1) ability to suppress ionization: (2) high permeability to gases so as to permit thorough and eflicient drying of the insulating medium; (3) high permeability to liquids so as to permit thorough and efdcient impregnation of the insulating medium; (4) chemical inertness with respect to adjacent materials with and without applied electric stress; (5) absence of gas adsorption so as to preclude the possibility of gas evolution under electric stress.

It is well known that the energy of electrical discharge from metallic conductors into dielectrics maybe greatly reduced by interposing, be-

tween the metal and dielectric, a layer which possesses high energy absorption characteristics by virtue of high dielectric loss.

I have discovered that this desirable result may be obtained by employing a layer of material which exhibits a high; work function, the work.

function being a measure of the electric energy required to detach electrons. An electron barrier consisting of.a layer of material which exhibits a high work function represents a far more eflicient means than the former of obtaining the benefits of this electrical phenomenon, because it accomplishes the ionization suppressing function through a low loss impedance rather than through a high loss resistance. The practical electric device embodying my electron ban'ier in its preferred form, after which I shall point out in the claims those features which I believe'to be new and of my own invention.

In the drawing: Figure 1 is a part section showing my invention as applied to an electric device.

Figure 2 is a graph of a charging current wave of an electric device in which my invention is not used.

Figure 3 is a similar graph of a charging current wave of an electric device in which my invention is used.

Figure 4 is a curve showing how drying tim of insulation may be affected by the use of my invention.

In the carrying out of my invention I place my electron barrier ll between the insulation i2 and the conductors l3.

While the structure details of the several electric devices in which my electron barrier is employed in the carrying out of my invention vary greatly, the electron barrier is always interposed between the conductor and its associated insulation.

For the purposes of this invention an intermediate value of work function, several times as great as that for metals, is required, since the aim is to reduce and equalize electron emission rather than to dam it. Due to experimental difllculties in determining work function in terms 01 volts, I have adopted a more practical procedure using a cathode-ray oscillograph. Comparing, by

effect of such a layer is to suppress or reduce the destructive energy of electric discharges which occur in dielectrics at conductor surfaces, particularly in vacuous pockets, by substantially re-' stricting the charging current of an electric device to a low loss condensive path. This, of course, increases the life of the dielectric, particularly if subjected to occasional over voltages.

Work function is measured in volts, these volts representing the potential level that must be attained to produce electron emission in vacuo. For metals or carbon the value is between 3 and 6 volts, whereas for good insulation, it is substantially infinite up to the breakdown point.

this means, the high energy content of discharge from metal. with that from my electron barrier,

I find the latter is markedly less. Thus the oscillograms of Figures 2 and 3 were obtained by photographing the 60 cycle charging current waves, as reproduced by a cathode ray oscillograph. The oscillogram shown in Figure 2 is typical of the electric discharge across a gaseous void between a highly conducting metal conductor and a high resistivity, low-power-factor insulation. The oscillogram shown in Figure 3 The foregoing. and other features of my inven-.

tion will now be described in connection with the accompanying drawing forming part of this specification in which I have represented an is similarly typical of the discharge across a gaseous void between a highly conducting metal conductor covered with my electron barrier and ide possess electron barrier qualities of a very high order.

A sheet of the above-characteristics is particularly valuable for application to high voltage electric devices, the insulation of which in the course of manufacture first must be vacuum treated, then impregnated with-low power factor fluid. In manufacturing procedures of this description, every efiort must be made to eliminate, or, as much as is practical, reduce impedance to the flow of fluids, gaseous or liquid. Under the high vacuum employed, that is, one millimeter of mercury absolute or less, the gas and moisture which are sorbed in the dielectric assume highly enlarged volumetric dimensions. These quantities of fiuid nevertheless must be removed during the manufacturing process in the interest of ultimate operating success for the device; andin the interest of manufacturing economy, this gaseous fluid removal and the subsequent impregnation with liquid insulating fluid must be achieved within a minimum of time.

Impervious electron barriers would not permit attainment of minimum manufacturing time. For example, consider the vacuum drying characteristic of a three-conductor, 500,000 circular mil, sector paper cable for 15,000 volt service. This cable has 0.203 inch of paper insulation on each conductor, and the radius under lead sheath is- 1.11 inches, the ratio of radius noted to in sulation-thickness being 5.47. Reference to Figure 4, graph of the time-factor for drying of three conductor sector paper cable possessing an impervious covering over the conductors, shows that it takes 5.5 times as long to dry a cable of this construction as it does to drya cable with a por= ous conductor covering. The economic advan- ,tage of a porous construction over the impervious construction is unquestionable in this comparison.

High permeability to liquids is required to expedite impregnation. This is indicated by a capillarity test, and is discussed further below.

An electron barrier as envisaged above must necessarily be chemically inert with respect to the adjacent insulation lest the power factor or dielectric strength of the latter be injuriously affected, But mere chemical inertness is not sumcient, it being essential that this condition shall hold in the presence of electrical discharges such .as may occur in voids or oil pockets within the insulation. Such discharges may result in evolution of adsorbed gas if materials which possess an adsorptive characteristic be employed; the sheet to this specification must minimize this action.

The highly capillary, porous, chemically neutral, electron barrier, subject of this invention,

usually consists of between 50 and 60 percent by weight of powder supportedon a residuum of fibre mesh, as described in my copending application filed February 5, 1944, Serial No. 474,885;

now Patent No: 2,389,678, dated November 27, 1945.

The invention is not necessarily restricted to the three powders listed above but only a few carcass by a low work function. Also, it would be imtical mechanical usefulness of the. sheet, Significant improvement is achievedin two char,v

powders possess the characteristics requisite to the formation of an electron barrier sheet'of this kind. They are medium density materials with a specific gravity between 3 and 6, and are special fusion products of high temperature furnaces; By virtue of their treatment in manufacture, they exhibit a marked stability of structure and are inert toward insulating fluids.

Metallic powders may not be employed in substitution for the above-mentioned powders. both because of their relatively low work functions and because of the deleterious eflect which they have on the power factor and electrical stability of fluid insulating compound. The deleterious effect on insulating compound power factor is caused by the metal oxide which coats the surface of each particle of metal powder. Aluminum, copper, iron, and other metals commonly suggested for this purpose reactreadily with the oxygen of the atmosphere. Theamount of oxide in relation to weight of metal is magnified considerably by the high degree of subdivision in powder form and consequent increase in surface area, thus accounting for the extreme vitiating powers of metal powders toward fluid insulating compound.

Graphite, carbon black, and similar powders, likewise, may not be used assubstitutes for the higher gravity powders described above, because if used as the basic material, the resultant sheet would be mechanically weak and be characterized possible to prevent carbon from powdering out of a sheet of this formula, unless a heavy sizing were employed in the furnish. Sizing is doubly objectionable for paper of this type, first because it is extremely vitiating to fluid insulatingcompound, and second, because it renders the sheet impervious, thusladding the disadvantages of an If carbon in. any form is used in small proportions as a mere impervious sheet detailed above.

modifier of paper, it is open to other objections.

Thus, carbon blacks are not inertto fluids, but

sheet made as described in my copending appli-- cation are included in the following table.

Physical characteristics of six mil sheet. Made from furnish beaten to a. slowness of 300 cc. 1

' Electron bar- Normalriensheet sheet Tensile strength in lbs. per sq. in 4,200.. 9.000 Gurley air density in'seconds 48 200. Capillarity test'in seconds Less than 1.... 70

Study of the table reveals that at the expense of slightly over 50 per cent loss in tensile strength,

a loss which by no means jeopardizes the .pracacteristics which dominate the manufacturing time and operating efficiency of vacuum proc-.

essed, fluid impregnated electric devices. The

Gurley air density for a six milsheetis reduced from 200 to 48 seconds, which indicates that this sheet offers less than 25 per cent of the im- -5 pedance, to gaseous fluid flow that normal sheet does. The decrease in impedance which the sheet ofiers to liquid. impregnation is even more pronounced, being approximately one one-hundredth that of the normal sheet. Therefore,

this sheet may be included in the dielectric of vacuum treated, fluid impregnated electric-devices, such as. electric cables and condensers,

manu-.

without furthermodification of standard facturing procedures.

I wish it distinctly understood that my electric device with electron barrier herein described and illustrated is in the form in which I desire tion a conductor, high voltage insulation therefor of the insulating oil impregnated fibrous type,

an insulating oil-pervious electron barrier containing sub-microscopic silicon carbide having a 25 1,027,004

chemical formula of SiCinterposed betweenthe conductor and the insulation, whereby the insulation is protected from electric iischarges fro the conductor.

2. An electric device comprising in combination, a conductor, insulation therefor, said in sulation impregnated with low power factor insulating fluid, an electron barrier interposed between the conductor and the insulation and applied to the surface of the insulation consisting of sub-microscopic silicon carbide having a chemical formula of SiC.

EbWIN JosEPri -MERRELL.

REFERENCES CITED The following references are -of record in the file of this patent:

20 'UNITED STA'I'IElS PATENTS Number Name Date 1,814,102 Weisetv July 14, 1931 1,784,989 Hill Dec. 16, 1930 1,084,199 Egly Jan. 13, 1914 Schwerin May 21, 1912 

